Throughout this specification, including in the claims, the phrase "grazing incidence angle" is employed to denote an incidence angle in the range from 80 degrees to 90 degrees (where an incidence angle of zero degrees denotes normal incidence).
Waveguides and grazing incidence optics are employed in many commercial systems to reflect visible radiation (such as HeNe laser radiation), or infrared radiation (such as CO.sub.2 laser radiation) and longer wavelength electromagnetic radiation, incident with a grazing incidence angle. When a purely metallic reflector is employed to reflect electromagnetic radiation at grazing incidence angles, the components of the radiation having S-polarization are reflected with high efficiency, but the components having P-polarization are reflected with low efficiency.
For example, FIG. 1 represents the reflectivity of gold (or silver) as a function of incidence angle, to a beam of CO.sub.2 laser (infrared) radiation. FIG. 1 shows that the reflectivity of gold (and silver) to the P-polarized component of the CO.sub.2 laser beam decreases substantially with increasing incidence angle, but that the reflectivity to the S-polarized component of the CO.sub.2 laser beam does not exhibit significant angular dependence. It will be appreciated by inspecting FIG. 1 that after a CO.sub.2 laser beam undergoes multiple reflections (at grazing incidence) from a purely metallic waveguide, substantially the entire P-polarized component of the beam may be lost, undesirably causing a beam power loss of about 50% during propagation through the waveguide.
FIG. 2 represents the reflectivity of silver as a function of incidence angle to visible radiation (orange light having wavelength 0.6 micrometer), and FIG. 3 represents the reflectivity of gold as a function of incidence angle to the same visible radiation. FIG. 2 shows that the reflectivity of silver to the P-polarized component of the radiation decreases slightly with increasing incidence angle, but that silver's reflectivity to the S-polarized component of the radiation does not exhibit significant angular dependence. FIG. 3 shows that the reflectivity of gold to the P-polarized component of the 0.6 micrometer visible radiation depends strongly on incidence angle (although the minimum reflectivity occurs at a lower incidence angle than in FIG. 1), and that gold's reflectivity to the S-polarized component of the radiation increases with increasing incidence angle.
In an effort to address the problem of decreased reflectivity of metal (to visible radiation) in certain incidence angle ranges, it has been proposed to deposit stacks of alternating high and low refractive index dielectric layers, each having a matched optical thickness of a quarter-wavelength, on metal to increase the metal's reflectivity to visible radiation. This technique efficiently increases reflectivity to normally incident visible radiation (having both P- and S-polarization), but it also increases reflectivity to visible radiation (of both polarizations) that is incident at grazing incidence angles (although with lower efficiency). However, if the difference between reflectivity to P-polarized and S-polarized radiation is large (for example, at a grazing incidence angle), an impractically large number of quarter-wave layers are required in a stack to achieve adequate reflectivity to both P-polarized and S-polarized components at grazing incidence angles.
Another limitation of the prior art technique described in the previous paragraph is that a stack of layers, each having quarter-wave thickness at a visible wavelength (such as a HeNe laser wavelength), will have no significant effect on infrared (or longer wavelength) radiation incident thereon. Thus, although a reflector coated with a multi-layer dielectric stack may have adequate reflectivity to visible radiation at grazing incidence, it will generally not have adequate reflectivity to infrared (or longer wavelength) radiation in the same grazing incidence angle range.
Waveguides and grazing incidence optics have been employed in commercial systems to reflect infrared radiation (such as CO.sub.2 laser radiation) and longer wavelength radiation incident at grazing incidence angles. For example, U.S. Pat. No. 4,805,987, issued Feb. 21, 1989, and U.S. Pat. No. 5,005,944, issued Apr. 9, 1991, to Laakman, et al., disclose hollow lightpipes and lightpipe tip members that are highly reflective of CO.sub.2 laser radiation at grazing incidence angles. Each of these hollow members consists of a housing (said to consist preferably of flexible metal) and a highly reflective coating on the housing. The reflective coating is a dielectric material (such as silicon carbide) having refractive index with a real part less than one, and having sufficient thickness to assure bulk absorption properties.
However, conventional reflectors that are highly reflective of long wavelength electromagnetic radiation (including those described in U.S. Pat. Nos. 4,805,987 and 5,005,944) over a range of incidence angles (including grazing incidence angles) have not also been highly reflective of substantially shorter wavelength radiation (e.g., visible radiation) over the same range of incidence angles.
Moreover, conventional reflectors of the type described in U.S. Pat. Nos. 4,805,987 and 5,005,944 are difficult to form into hollow waveguides (or lightpipes). This is because when flat substrates coated with commonly used reflective coatings of the type described in U.S. Pat. Nos. 4,805,987 and 5,005,944 are rolled to form tubes, stresses on the coatings often cause the coatings to crack (as explained at column 5, lines 50-60 of U.S. Pat. No. 4,805,987). Use of reflective coating materials having high ductility (such as lead fluoride) may avoid the cracking problem, but such ductile materials are unsuitable for many applications because they are toxic.
It is also difficult to sputter (or otherwise deposit) a reflective coating (of the type described in U.S. Pat. Nos. 4,805,987 and 5,005,944) on a pre-formed hollow waveguide housing, particularly in the typical case that the hollow housing has very small diameter (e.g., one millimeter) and very long length (e.g., one meter). It is particularly difficult to deposit reflective coatings on such a hollow housing in a manner providing precise control of the thickness of the deposited layer.